isalnum()

Updated: May 06, 2022

Test a character to see if it's alphanumeric

Synopsis:

#include <ctype.h>

int isalnum( int c );

Arguments:

c
The character you want to test. This must be representable as an unsigned char or be EOF; the behavior for other values is undefined. Because this argument is interpreted as an int, to avoid sign extension on character values greater than 0x7F, you must cast the argument to the unsigned data type; otherwise, the function will behave unpredictably.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The isalnum() function tests if the argument c is an alphanumeric character (a to z, A to Z, or 0 to 9). An alphanumeric character is any character for which isalpha() or isdigit() is true.

Returns:

Nonzero if c is a letter or decimal digit; otherwise, zero.

Examples:

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>

int main( void )
{
    if( isalnum( (unsigned)getchar() ) ) {
        printf( "That's alpha-numeric!\n" );
    }
    
    return EXIT_SUCCESS;
}

Classification:

ANSI, POSIX 1003.1

Safety:  
Cancellation point No
Interrupt handler Yes
Signal handler Yes
Thread Yes

Caveats:

The result is valid only for char arguments and EOF.